The present invention is a sheet metal screw. More particularly, the invention is a sheet metal screw for connecting thin layers of material, which screw will resist stripout more effectively than other known sheet metal screws. "Stripout" is the tendency of a screw to become loosened from the position into which it has been tightened by the application of torque in excess of the driving torque, to the point of failure of the clamping load, i.e., overtightening.
Barth U.S. Pat. No. 4,516,893 teaches a rotary fastener having a serrated helical ramp forged into the undersurface of the head. The ramp extends part way through the mating holes in two pieces of sheet metal being fastened together and allegedly engages the lower sheet. However, it is clear that the configured ramp will tend to move the screw from side to side during tightening, and can give a false indication of tightness as the screw is being installed. Barth, however, attains optimum strip-out torque uniformly as added torque is applied.
Other rotary fastener patents which may be of interest to the reader include: Loretan U.S. Pat. No. 3,258,047; Churla U.S. Pat. No. 3,761,867; Wagner U.S. Pat. No. 3,960,048; Wagner U.S. Pat. No. 4,238,165; and Temple, Jr., U.S. Pat. No. 4,295,767.